1. Field of the Invention
The invention relates to a motor controller for driving a brushless motor without using sensors, as well as to a vehicular steering system which uses that motor controller. A brushless motor is used as a source for generating steering force or steering assist force in an electric power steering system or other vehicular steering system, for example.
2. Description of the Related Art
A motor controller for controlling (i.e., driving) a brushless DC motor typically configured to control the supply of motor current according to output from a position sensor that detects the rotational position of a rotor. However, in addition to the position sensor not being very environmentally resistant, expensive position sensors and the wiring they require make it difficult to reduce costs as well as the size of the motor controller. Therefore, a sensorless drive system for driving a brushless DC motor without using a position sensor has been proposed. A sensorless drive system is a system that estimates the magnetic pole phase (i.e., the electrical angle of the rotor) by estimating the induced voltage generated by rotation of the rotor.
When the rotor is stopped or rotating extremely slowly, the induced voltage is unable to be estimated so the magnetic pole phase is estimated by another method. More specifically, as shown in FIG. 2A, high-frequency probe voltage is applied to a U-phase stator winding 51, a V-phase stator winding 52, and a W-phase stator winding 53 to form a high-frequency voltage vector (of a consistent magnitude) that rotates in the direction of rotation of a rotor 50 around the origin of CCP coordinates which are fixed coordinates with their origin at the center of rotation of the rotor 50. The high-frequency voltage vector is a voltage vector that rotates at sufficiently high speed relative to the rotation speed of the rotor 50. As this high-frequency voltage vector is applied, a current flows through the U-phase stator winding 51, the V-phase stator winding 52, and the W-phase stator winding 53. The current vector that indicates the magnitude and direction of the current of these three phases on the αβ coordinate system rotates about the origin.
The inductance of the rotor 50 has a different value at a d-axis, which is a magnetic pole axis in the direction of magnetic flux, than it does at a q-axis which is orthogonal to the d-axis (i.e., an axis in the direction of torque). Therefore, the magnitude of the current vector becomes larger in the direction closer to the d-axis and smaller in the direction closer to the q-axis. As a result, the terminus of the current vector makes an elliptical trajectory 55 with the d-axis direction of the rotor 50 as the major axis on the αβ coordinate system, as shown in FIG. 2B.
Therefore, the magnitude of the current vector has local maximum values in the directions of the N-pole and S-pole of the rotor 50. That is, the magnitude of the voltage vector changes as shown in FIG. 3B, while the magnitude of the current vector has two local maximum values in a single cycle, as shown in FIG. 3A. In this case, when the magnitude of the voltage vector is sufficiently large, the effect of magnetic saturation of the stator causes the inductance on the N-pole side of the rotor 50 to be smaller than the inductance on the S-pole side of the rotor 50, so the current vector in the direction of the N-pole assumes the highest value (see curve L1).
Therefore, a sufficiently large high-frequency voltage vector is applied to identify the local maximum current vector corresponding to the N-pole, after which a smaller high-frequency vector is applied, and the phase of the rotor 50 can then be estimated based on the local maximum value of the current vector. More specifically, the phase angle (electrical angle) θ of the rotor 50 can be obtained as θ=Tan−1 (Iβ/Iα) from an α-axis component Iα and a β-axis component Iβ of the current vector when it is at the maximum value.
Meanwhile, Japanese Patent Application Publication No. 2004-343963 (JP-A-2004-343963) describes a method for estimating the rotation angle of a rotor using induced voltage. However, the position estimating method which uses high-frequency probe voltage is not suitable for use in the high speed region where the rotor position can be estimated using induced voltage because efficiency may decrease as a result of the high-frequency probe voltage being applied, which may adversely affect the control of the motor.